Projectors are known that have a light source an optical system a micro-display, whereby the optical system includes a projection lens for projecting an image onto a screen.
Contrast has a strong impact on the perceived quality of images [30], [31], [32]. In a micro-display based projection display multiple factors are determining the contrast of the projected image. In the dark state there is always a small amount of light leakage through the optical system, resulting in a black level with a finite luminance level. This can be caused by various mechanisms and depends on the type of micro-display technology e.g. limited extinction ratio, skew rays, birefringence, light scattering, diffraction etc.
Various prior approaches for reducing the black level have been proposed. In [26] a method to block diffracted light with both an aperture in the illumination ray bundle and the projection lens was proposed. The reduction of efficiency of the projector was minimized by an optimal shape and orientation of the illumination and projector apertures. In [28] an asymmetric aperture stop on the light path between two groups of lenses of the projection lens was proposed to block scattered light without blocking the desired projection light. In [29] an adjustable asymmetrical illumination aperture and an adjustable asymmetrical projection aperture was proposed to increase the grayscale resolution. In [27] a dynamic aperture to modulate the light intensity in the illumination path according a control signal being a function of the image brightness level was proposed.
The prior approaches for reducing the black level all suffer from a loss of luminance of the bright pixels.